Modeling and simulation of CO₂ absorption enhancement in hollow-fiber membrane contactors using CNT-water-based nanofluids

Absorption of CO₂ from a gas mixture containing CO₂ and nitrogen by water-based CNT nanofluids in gas-liquid hollow fiber membrane contactor was modeled and solved using COMSOL Multiphysics 5.4. The model assumed partial wetting of the membrane, along with diffusion in the axial and radial directions. In addition, Brownian motion and grazing effects were both considered in the model. The main contribution to the mass transfer resistance for the case of external diffusion-controlled adsorption is the stagnant liquid layer around the particles, despite the layer being very thin. Accordingly, the nanofluid flows in the lumen tube side of the hollow fiber membrane was modeled as a solid-free zone and dense solid phase. The simulations were performed using 7% wetting of the membrane thickness. The results showed a significant increase in CO₂ absorption with increasing concentration of carbon nanotubes (CNT). At a fixed inlet gas flow rate (20 L/h), increasing the CNT concentration from 0.1 wt.% to 0.25 wt.% increased the CO₂ removal from around 20% to 45%. Comparison of the model predictions with experimental data available in the literature confirmed the validity of the developed model.